1. Field of the Invention
The present invention relates to a lithographic projection apparatus and device manufacturing method.
2. Description of the Related Art
The term “patterning device” as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described above, the patterning device can include one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as set forth above.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (IC's). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay (juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as “alignment system”), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796.
Correct imaging of a pattern generated by a patterning device in a lithographic projection apparatus requires correct illumination of the patterning device. In particular, it is important that the intensity of illumination proximal the plane of the pattern, as generated by the patterning device, or proximal planes conjugate to the plane of the pattern, be uniform over the area of the exposure field. Also, it is generally required that the patterning device can be illuminated with off-axis illumination in a variety of modes such as, for example, annular, quadrupole or dipole illumination, to improve resolution. The use of such illumination modes is disclosed, for example, in U.S. Pat. No. 6,671,035. The illumination modes are obtained, for example, by providing a corresponding predetermined intensity distribution in a pupil of the illumination system.
To meet the above-mentioned requirements, the illumination system of a lithographic projection system is generally quite complex. A typical illumination system might include: shutters and attenuators configured to control the intensity of the beam output by the source, which might be a high pressure Hg lamp or an excimer laser; a beam shaping element such as, for example, a beam expander for use with an excimer laser radiation beam configured to lower the radiation beam divergence; a zoomable axicon pair and a zoom lens configured to set the illumination mode and parameters (collectively referred to as a zoom-axicon); an integrator, such as a quartz rod, configured to make the intensity distribution of the beam more uniform; masking blades configured to define the illumination area; and imaging optics configured to project an image of the exit of the integrator onto the patterning device. For simplicity, the plane of the pattern generated by the patterning device, and planes conjugate to this plane in the illumination system and the projection system may be referred to hereinafter as “image” planes.
The illumination system may also include elements configured to correct non-uniformities in the illumination beam at or near image planes. For example, the illumination system may include diffractive optical elements to improve the match of the beam cross-section proximal the entrance face of the integrator rod with the shape of the entrance face. A diffractive optical element typically includes an array of microlenses, which may be Fresnel lenses or Fresnel zone plates. Improving the match alleviates the problem of field dependent lithographic errors occurring in the patterned layer. The matching may hereinafter be referred to as “filling” of the integrator entrance face. A diffractive optical element may also be positioned, for example, in front of a beam shaping element, such as a zoom-axicon, to transform the angular distribution of radiation provided by an excimer laser beam into a predetermined angular distribution of radiation to generate a desired illumination mode. Illumination systems as discussed above are disclosed, for example, in U.S. Pat. Nos. 5,675,401 and 6,285,443.
The illumination system may also include, for example, a filter partially transmissive to radiation of the beam with a predetermined spatial distribution of transmittance, immediately before the plane of the pattern, to reduce spatial intensity variations.
The illumination systems discussed above still suffer from various problems, however. In particular, various elements that are used, especially diffractive optical elements and quartz-rod integrators, can introduce an anomaly of intensity distribution in a plane perpendicular to the optical axis of the radiation system or the projection system. For example, in a plane proximal a pupil of the radiation system or the projection system, either the beam cross-section may be elliptical rather than circular, or the beam intensity distribution within the beam cross-section may, for example, be elliptically symmetric rather than circularly symmetric. Both types of errors are referred to as “ellipticity of the beam” or simply as “ellipticity error,” and typically lead to specific lithographic errors in the patterned layers. In particular, a patterned feature occurring in directions parallel to both the X and Y direction may exhibit, in the presence of ellipticity of the beam, different sizes upon exposure and processing. Such a lithographic error is usually referred to as H−V difference. Also, a diffractive optical element used to improve filling of the integrator is generally only optimum for one setting of the zoom-axicon. For other settings, the integrator entrance face may be under-filled (the beam cross-section is smaller than the integrator entrance face), leading to substantial field dependent H−V difference. For other settings, the integrator entrance face may also be over-filled, leading to energy wastage. Additionally, 157 nm excimer lasers and other excimer lasers tend to have large divergence differences in X and Y directions which cannot be completely resolved using beam expander lenses while keeping the shape of the beam within acceptable dimensions.
Ellipticity errors in the beam may also be caused by subsequent elements, such as the mask and elements of the projection system. Current lithographic apparatus do not correct or compensate for ellipticity errors introduced into the beam by elements subsequent to the illumination system.
In a lithographic apparatus, it is important that illumination of the patterning device is uniform in field and angle distribution and, for illumination modes such as dipole and quadrupole illumination, all poles are equal. To achieve this, an integrator is provided in the illumination system. In a lithographic apparatus using UV or DUV exposure radiation the integrator may take the form of a quartz rod or a so-called fly's eye lens. A fly's eye lens is a lens built up of a large number of smaller lenses in an array which creates a correspondingly large number of images of the source in a pupil plane of the illumination system. These images act as a virtual, or secondary, source. However, when using EUV exposure radiation the illumination system must be constructed from mirrors because there is no known material suitable for forming a refractive optical element for EUV radiation. In such an illumination system, a functional equivalent to a fly's eye lens can be provided using faceted mirrors, for instance as described in U.S. Pat. Nos. 6,195,201, 6,198,793 and 6,452,661. These documents describe a first, or field, faceted mirror which focuses a plurality of images, one per facet, on a second, or pupil, faceted mirror which directs the light to appropriately fill the pupil of the projection system. It is known from UV and DUV lithography that imaging of different types of mask patterns can be improved by controlling the illumination settings, e.g. the filling ratio of the pupil of the projection system (commonly referred to as σ) or the provision of special illumination modes such as annular, dipole or quadrupole illumination. More information on illumination settings can be obtained from U.S. Pat. No. 6,452,662 and U.S. Pat. No. 6,671,035.
In an EUV lithographic apparatus with a fly's eye integrator, these illumination settings can be controlled by selectively blocking certain of the pupil facets. However, because the source position and size on each facet is not exactly known and not stable, it is necessary to block off whole facets at a time, rather than partial facets. Thus, only relatively coarse control of illumination settings is possible. Also, to provide an annular illumination setting it is necessary to obscure the innermost pupil facets and when positioning a masking blade over an inner facet it is difficult to avoid partially obscuring one or more of the outer facets.
Referring to FIGS. 14a and 14b, a pupil plane includes a plurality of overlapping pupils. The overlapping pupils produce a field plane. As shown in FIGS. 14a and 14b, a square field plane is formed by four overlapping cones of radiation in the pupil plane. In the pupil plane, the pupil shape can be corrected for all field positions with transmission filters. The pupil distribution cannot be influenced over the field in a controlled manner. Only a pupil correction that is constant over the field can be applied. In the field plane, the uniformity profile can be changed for all angles with transmission filters. Transmission filters can only change the transmission for all angles.
In order to make field dependent corrections to the pupil distribution, the correction must take place between the pupil plane and the field plane. The difficulty is that the pupil of one field point overlaps with the pupil of an adjacent field point. In an intermediate plane, the left portion of a pupil corresponding to a right field point overlaps with the right portion of a pupil corresponding to a left field point. Accordingly, it is not possible to correct one portion of the pupil for one field point without affecting the pupil for another field point.